Optical information recording medium

ABSTRACT

An optical information recording medium which makes it possible to reduce the noise level and improve the C/N ratio. An optical information recording medium has a recording layer formed on a substrate, for having a laser beam irradiated thereto for recording and reproduction of record data. The recording layer includes a first sub-recording film and a second sub-recording film. The first sub-recording film is formed of a first material containing Si as the main component. The second sub-recording film is formed of a second material containing Zn as the main component and having Cu added thereto, and disposed in the vicinity of the first recording film. The laser beam is irradiated to the recording layer via a light transmitting layer formed in a manner covering the recording layer side.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical information recording mediumconfigured to be capable of recording and reproducing record data byirradiating a laser beam to a recording layer formed on a substrate.

2. Description of the Related Art

As an optical information recording medium of this kind, the presentassignee has proposed a single-sided write-once optical informationrecording medium in Japanese Patent Application No. 2002-105994. Theoptical information recording medium is comprised of a substrate formedof a polycarbonate or the like by an injection molding method, and areflective layer, a recording layer, a protective layer, and a coveringlayer, deposited on the substrate. The reflective layer reflects a laserbeam irradiated from the covering layer side when record data isrecorded and reproduced, and is in the form of a thin film made of ametal, such as Au, Ag, or Al, or a mixture of selected ones of thesemetals. The recording layer is provided in a manner sandwiched between afirst protective film and a second protective film constituting theprotective layer. The recording layer is formed by sequentiallydepositing a second sub-recording film and a first sub-recording film onthe second protective film in the mentioned order. In this case, thefirst sub-recording film is formed of a highly reflective metal, such asAl, Cu, Ag, or Au, and the second sub-recording film is formed of areactive metal, such as Sn, Te, Sb, Ge, Si, or C, or a semimetal. On theother hand, the first and second protective films (dielectric layers) ofthe protective layer prevent oxidation of the sub-recording films duringirradiation of the laser beam, and deformations of the substrate and thecovering layer, and is formed of a dielectric material, such asZnS+SiO₂. The covering layer (light-transmitting layer) is formed ofe.g. a light-transmitting resin applied by spin-coating, and provides anoptical path of the laser beam while preventing the layers on thesubstrate from being scratched.

In recent years, optical information recording media are desired to havethe capability of recording and reproducing a large amount of recorddata at a high speed (in a short time period). Accordingly, the opticalinformation recording medium of the above-mentioned kind is required toenhance recording density of record data, and to meet the requirement,the diameter of a beam spot of a laser beam used for recording orreproduction of record data tends to be reduced. More specifically,record data are recorded and reproduced using a pickup which is equippedwith an objective lens having a numerical aperture (NA) of not less than0.7 (e.g. a numerical aperture (NA) of approximately 0.85), and iscapable of emitting a laser beam having a wavelength (λ) of not morethan 450 nm (e.g. a wavelength (λ) of approximately 405 nm). However, asthe numerical aperture (NA) is larger, the allowable angle range (i.e.tilt margin) of the optical axis of the laser beam with respect to theoptical information recording medium becomes smaller. Therefore, in thecase of the construction of the optical information recording medium,such as a conventional general type, in which the laser beam emittedfrom the pickup is irradiated through the substrate to the recordingmedium, the substrate has a rather large thickness of e.g. approximately1 mm, and therefore it is difficult to obtain a desired tilt margin. Toovercome this problem, the optical information recording medium proposedby the present assignee employs the construction which allows the laserbeam to be irradiated through the covering layer having a thickness ofapproximately 100 μm, which is formed in a manner covering the recordinglayer, to the recording layer, whereby a tilt margin is obtained whichis large enough to stably record and reproduce record data. In thiscase, to suppress coma of the laser beam as well, it is preferable toreduce the thickness of the covering layer (e.g. to a value of e.g. 100μm).

To record data on the optical information recording medium (forming pitsaccording to contents of record data), a laser beam adjusted to arecording power is irradiated to the recording layer. The irradiation ofthe laser beam causes the two sub-recording films to be mixed with eachother (atomic arrangement thereof to be changed) to form recordedportions (pits). On the other hand, to reproduce the record datarecorded on the optical information recording medium (determine whetheror not each pit exists), a laser beam adjusted to a reproducing power isirradiated to the recording layer. The recorded portions and theunrecorded portions are different in optical constant therebetween, sothat the irradiation of the laser beam to these portions revealsdifferent values of reflectance thereof. Therefore, by detecting thedifference, it is possible to determine the existence of each pit in therecording layer (whether or not the pit exists in the recording layer)to thereby reproduce the record data.

The present inventors have studied the above-descried opticalinformation recording medium, and found the following points to beimproved: the optical information recording medium employs theconstruction which allows the laser beam to be irradiated through thecovering layer formed in a manner covering the recording layer, to therecording layer. In this case, when the optical information recordingmedium is produced, there are the reflective layer, the secondprotective layer, the second sub-recording film, the first sub-recordingfilm, the first protective layer, and the covering layer, sequentiallydeposited on the substrate in the mentioned order. In this case, asshown in FIG. 4, the reflective layer (3) formed on the substrate (2)which is formed such that the surface of each guide track (upper surfaceof land and bottom surface of groove: upper surface of the substrate (2)as viewed in FIG. 4) is flat has a slightly rough surface compared withthe surface of the substrate (2). It should be noted that in FIG. 4, theguide track (groove and land) is omitted from illustration for ease ofunderstanding the multilayer structure. Further, the second protectivelayer (5 b) formed on the slightly rough surface of the reflective layer(3) has a rougher surface than that of the reflective layer (3).Therefore, after the layers i.e. the reflective layer (3) to thecovering layer (6), are sequentially deposited, the surface of eachlayer becomes rougher as the layer is more distant from the substrate(2) (i.e. as closer to incidence plane of the laser beam). FIG. 4 showsthe progressively roughened conditions of the respective surfaces of thelayers in an exaggerated manner for purposes of illustration.

In the above case, when the upper surface of the second protective layer(5 b) (interface between the second protective film (5 b) and the secondsub-recording film (4 b)), the upper surface of the second sub-recordingfilm (4 b) (interface between the second sub-recording film (4 b) andthe first sub-recording film (4 a)), and the upper surface of the firstsub-recording film (4 a) (interface between the first sub-recording film(4 a) and the first protective film (5 a)) are significantly rough, thelevel of noise contained in a reproduction signal (noise level) is high,so that the C/N ratio is lowered. Particularly, in the case of theoptical information recording medium (1) of a type to which a laser beamhaving a short wavelength is emitted from an objective lens having alarge numerical aperture to record and reproduce record data, the laserbeam irradiated to the sub-recording films (4 a and 4 b) forms a beamspot having a very small diameter, so that the surface smoothness of thesecond sub-recording film (4 b) has large influence on the noise levelof the reproduction signal and the value of the C/N ratio. Therefore,depending on the reproduction speed, it can be difficult to accuratelyread record data recorded on the optical information recording medium(1), since the surface of each layer thereof tends to be formed to berougher as the layer is closer to the incidence plane of the laser beam.To overcome this problem, it is desirable to reduce the noise level andimprove the C/N ratio.

SUMMARY OF THE INVENTION

The present invention has been made to solve the problems describedabove, and a main object thereof is to provide an optical informationrecording medium which makes it possible to reduce the noise level andimprove the C/N ratio.

To attain the above object, the present invention provides an opticalinformation recording medium for recording and reproducing record data,comprising a substrate, a recording layer formed on the substrate, forhaving a laser beam irradiated thereto for recording and reproduction ofthe record data, and a light transmitting layer formed in a mannercovering the recording layer, wherein the recording layer includes afirst recording film formed of a first material containing Si as a maincomponent, and a second recording film formed of a second materialcontaining Zn as a main component and having Cu added thereto, thesecond recording film being formed in the vicinity of the firstrecording film, and wherein the laser beam is irradiated to therecording layer from a light transmitting layer side. It should be notedthat in the present invention, the term “main component” is intended tomean a component which has the largest composition ratio (at %: atomicpercentage) of a plurality of elements constituting a material forforming a film or a layer.

With the arrangement of this optical information recording medium, thefirst recording film is formed by using a first material containing Sias the main component, and the second recording film is formed in thevicinity of the first recording film by using a second materialcontaining Zn as the main component and having Cu added thereto. Theaddition of Cu makes it possible to improve the smoothness of thesurface of the second recording film, thereby significantly lowering thenoise level. Therefore, the C/N ratio can be enhanced, whereby recorddata can be reliably reproduced.

Preferably, the second material has Cu added thereto in an amount notless than 10 at % and less than 50 at %. With this arrangement of thepreferred embodiment, it is possible to sufficiently lower the noiselevel in a frequency band which is of interest in the use of the presentoptical information recording medium. Therefore, an optical informationrecording medium can be provided which is capable of reliablyreproducing record data.

More preferably, the second material has Cu added thereto in an amountnot less than 16.5 at % and not more than 36 at %. With this arrangementof the preferred embodiment, it is possible to further reduce the noiselevel, so that record data can be more reliably reproduced.

Preferably, the recording layer is configured such that the first andsecond recording films are in contact with each other. With thisarrangement of the preferred embodiment, when the laser beam adjusted toa recording power is irradiated to the recording layer, the first andsecond recording films can be easily mixed with each other to therebyform the recorded portions.

Preferably, the recording layer is formed such that a total of athickness of the first recording film and a thickness of the secondrecording film is not less than 2 nm and not more than 30 nm. With thisarrangement of the preferred embodiment, the C/N ratio can be fullyenhanced, whereby record data can be reliably reproduced.

Preferably, the optical information recording medium includes a firstdielectric layer formed between the light transmitting layer and therecording layer, and a second dielectric layer formed between thesubstrate and the recording layer. With this arrangement of thepreferred embodiment, it is possible to configure the recording layer tobe sandwiched between the first and second dielectric layers, andtherefore, it is possible to prevent thermal deformations of thesubstrate and the light transmitting layer during irradiation of thelaser beam (during formation of the recorded portions). Further, sincecorrosion of the recording layer can be prevented, it is possible tostore record data for a long time period in a manner such that therecord data can be normally reproduced.

More preferably, the optical information recording medium includes areflective layer formed between the substrate and the second dielectriclayer. With this arrangement of the preferred embodiment, since theeffect of multi-beam interference is further increased, it is possibleto further increase the difference in optical reflectance between therecorded portions and the unrecorded portions, whereby record data canbe more reliably reproduced.

It should be noted that the present disclosure relates to the subjectmatter included in Japanese Patent Application No. 2003-208549 filed onAug. 25, 2003, and it is apparent that all the disclosures therein areincorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention will beexplained in more detail below with reference to the attached drawings,wherein:

FIG. 1 is a cross-sectional view showing the construction of an opticalinformation recording medium according to an embodiment of the presentinvention;

FIG. 2 is a characteristics diagram showing the relationship between theamount of Cu added to a material for forming a second sub-recordingfilm, and the noise level of a reproduction signal;

FIG. 3 is a characteristic diagram showing the relationship between thethickness of the recording layer (layer thickness) and the C/N ratio ofthe reproduction signal; and

FIG. 4 is a cross-sectional view showing the arrangement of an opticalinformation recording medium proposed by the present assignee.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention will now be described in detail with reference to theaccompanying drawings showing a preferred embodiment thereof.

First, a description will be given of the construction of an opticalinformation recording medium 1 according to the present invention.

The optical information recording medium 1 is a single-sidedsingle-layered optical disk of a write-once type, having an outerdiameter of approximately 120 mm and a thickness of 1.2 mm, andconfigured to be capable of recording and reproducing record data, usinga blue-violet laser beam (hereinafter referred to as the “laser beam”) Lhaving a wavelength (λ) not less than 380 nm and not more than 450 nm(e.g. 405 nm), emitted from an objective lens having a numericalaperture (NA) of not less than 0.7 (e.g. approximately 0.85). Morespecifically, as shown in FIG. 1, the optical information recordingmedium 1 is comprised of a reflective layer 3, a second dielectric layer5 b, a recording layer 4, a first dielectric layer 5 a, and a lighttransmitting layer 6, sequentially deposited on the substrate 2 in thementioned order. Further, the optical information recording medium 1 hasa central portion thereof formed with a central hole for mounting(clamping) the same on a recording/reproducing apparatus.

The substrate 2 is in the form of a disk with a thickness ofapproximately 1.1 mm, made e.g. of a polycarbonate resin by theinjection molding method. In this case, the substrate 2 can also beformed by any suitable one of various substrate-forming methods, such asthe 2P method. Further, one surface (upper surface as viewed in FIG. 1)of the substrate 2 is formed with grooves and lands extending helicallyfrom a central portion of the substrate 2 toward the outer peripherythereof. In this case, the grooves and the lands function as guidetracks for recording and reproducing data on and from the recordinglayer 4. Therefore, to enable accurate tracking to be performed, it ispreferable to form grooves between the lands, for example, such thatthey have a depth not less than 10 nm and not more than 40 nm, and apitch not less than 0.2 μm and not more than 0.4 μm. Further, theoptical information recording medium 1 is configured such that the laserbeam L is to be irradiated thereon from the light transmitting layer 6side when data is recorded or reproduced. Therefore, the substrate 2 isnot required to have a light transmitting property, i.e. be transparent,so that the optical information recording medium 1 has more options forselecting materials for forming the substrate 2 than the existinggeneral optical information recording media (e.g. CD-R). Morespecifically, the material for forming the substrate 2 is not limited tothe above-mentioned polycarbonate resin, but resin materials, such as anolefin resin, an acrylic resin, an epoxy resin, a polystyrene resin, apolyethylene resin, a polypropylene resin, a silicone resin, afluorocarbon resin, an ABS resin, and an urethane resin, as well asglass and ceramic materials can be employed as the substrate-formingmaterial. However, it is preferable to employ one of the resin materialsthat are easy to mold and relatively inexpensive, such as thepolycarbonate resin and the olefin resin.

The reflective layer 3 reflects the laser beam L irradiated thereon viathe light transmitting layer 6 when record data is reproduced, and ismade of any of metal materials, such as Mg, Al, Ti, Cr, Fe, Co, Ni, Cu,Zn, Ge, Ag, Pt, and Au, or alloys containing selected ones of them (e.g.AgNdCu=98:1:1, and AgPdCu=98:1:1) such that it has a thickness not lessthan 10 nm and not more than 300 nm. In this case, to reflect thenecessary and sufficient amount of the laser beam L, it is preferable todefine the thickness of the reflective layer 3 to be not less than 20 nmand not more than 200 nm (e.g. 100 nm). Further, metal materials, suchas, Al, Au, Ag, Cu, and the metal materials such as an alloy of Ag andCu, have a high reflectance, and therefore it is preferable to use ametal material containing at least one of these metals as the materialfor forming the reflective layer 3.

The first dielectric layer 5 a and the second dielectric layer 5 b (alsoreferred to as the “dielectric layers 5” when they are not distinguishedfrom each other) correspond to first and second dielectric layers in thepresent invention, respectively, and are formed such that they sandwichthe recording layer 4. The dielectric layers 5 prevent (reduce)corrosion of the recording layer 4, thereby preventing degradation ofrecord data, and at the same time prevent thermal deformations of thesubstrate 2 and the light transmitting layer 6 during recording ofrecord data to thereby prevent deterioration of jitter characteristics.Further, the dielectric layers 5 also serve to increase the amount ofchange in the optical characteristics between recorded portions havingrecord data recorded thereon (portions of the recording layer, havingpits formed thereon) and unrecorded portions having no record datarecorded thereon (portions of the recording layer, having no pits formedthereon) by the effect of multi-beam interference. In this case, toincrease the amount of change in the optical-characteristics, it ispreferable to form the dielectric layers 5 using a dielectric materialhaving a high index of refraction (n) in the wavelength region of thelaser beam L. Further, when the laser beam L is irradiated, if anexcessively large amount of energy is absorbed by the dielectric layers5, recording sensitivity of the recording layer 4 is reduced. Therefore,it is preferred to form the dielectric layers 5 using a dielectricmaterial having a small extinction coefficient (k) in the wavelengthregion of the laser beam L to thereby prevent the reduction of therecording sensitivity.

More specifically, from the viewpoint of prevention of thermaldeformations of the substrate 2 and the light transmitting layer 6, andenhancement of protecting characteristics of the dielectric layers 5 forprotecting the recording layer 4 as well as obtaining the sufficienteffect of multi-beam interference, it is preferable to employ adielectric material having a light transmitting property, such as any ofAl₂O₃, AlN, ZnO, ZnS, GeN, GeCrN, CeO₂, SiO, SiO₂, Si₃N₄, SiC, La₂O₃,TaO, TiO₂, SiAlON (mixture of SiO₂, Al₂O₃, Si₃N₄, and AlN), and LaSiON(mixture of La₂O₃, SiO₂, and Si₃N₄), any of oxides, nitrides, sulfides,and carbides of Al, Si, Ce, Ti, Zn, and Ta, and mixtures thereof, as thedielectric material for forming the dielectric layers 5. In this case,the first dielectric layer 5 a and the second dielectric layer 5 b canbe formed of the same dielectric material, or alternatively byrespective dielectric materials different from each other. Further, oneor both of the first dielectric layer 5 a and the second dielectriclayer 5 b can be configured to have a multilayer structure formed by aplurality of dielectric layers.

In the optical information recording medium 1 according to the presentinvention, the first dielectric layer 5 a and the second dielectriclayer 5 b are formed of a dielectric material mainly composed of amixture of ZnS and SiO₂ (preferably, molar ratio of ZnS:SiO₂=80:20),such that they have a thickness not less than 10 nm and not more than200 nm (e.g. 25 nm). In this case, the mixture of ZnS and SiO₂ has ahigh index of refraction (n), and a relatively small extinctioncoefficient (k) with respect to the laser beam L in the wavelengthregion ranging from 380 nm to 450 nm inclusive, which causes a moreconspicuous change in optical characteristics of the recording layer 4before and after recording of data thereon, and at the same timeprevents the recording sensitivity thereof from being degraded. Further,the thickness of each of the first and second dielectric layers 5 a and5 b is not limited to the examples described above, but when thedielectric layer has a thickness of less than 10 nm, it is difficult toobtain the aforementioned effects. Inversely, when the dielectric layerhas a thickness of more than 200 nm, it takes a long time to deposit thedielectric layer, which can sharply increase the manufacturing costs ofthe optical information recording medium 1, and further cause cracks inthe optical information recording medium 1 due to internal stresses ofthe first dielectric layer 5 a or the second dielectric layer 5 b.Therefore, it is preferable to define the thicknesses of the first andsecond dielectric layers 5 a and 5 b to be not less than 10 nm and notmore than 200 nm.

The recording layer 4 has optical characteristics thereof changed by thelaser beam L irradiated thereto during recording of record data so as tobe formed with recorded portions M (pits). The recording layer 4 isformed by two thin films, i.e. a second sub-recording film 4 b and afirst sub-recording film 4 a, sequentially deposited on the substrate 2in the mentioned order. In this case, the recording layer 4 is formedsuch that the two thin films are deposited in the order of the firstsub-recording film 4 a and the second sub-recording film 4 b from thelight transmitting layer 6 side (side closer to the incidence plane ofthe laser beam L). This enables the optical characteristics of therecording layer 4 to be sufficiently changed even with a laser beam Lrelatively small in power, thereby making it possible to reliably formthe recorded portions M. The first sub-recording film 4 a corresponds toa first recording film according to the present invention, and is in theform of a thin film made of a material (first material in the presentinvention) containing Si as the main component. By forming the firstsub-recording film 4 a using the material containing Si as the maincomponent, it is possible to fully enhance the C/N ratio of areproduction signal, as will be described hereinafter. In the embodimentof the present invention, the atomic percentage of Si to the wholematerial for forming the first sub-recording film 4 a is defined to benot lower than 95 at % (e.g. 99 at %).

Further, the second sub-recording film 4 b corresponds to a secondrecording film according to the present invention, and is in the form ofa thin film made of a material (second material in the presentinvention) containing Zn as the main component and having Cu addedthereto. In this case, since Zn is inexpensive, it is possible to fullyreduce the manufacturing costs of the optical information recordingmedium 1. In contrast, when the second sub-recording film 4 b is formedusing a material in which no Cu or the like is added to Zn, it isdifficult to improve the smoothness of the upper surface of the secondsub-recording film 4 b. This results in a very high level of noisecontained in the reproduction signal, causing a significant decrease inthe C/N ratio. On the other hand, when the second sub-recording film 4 bis formed of a material obtained by adding Cu to Zn, the smoothness ofthe upper surface of the second sub-recording film 4 b can be furtherincreased. This makes it possible to decrease the noise level of thereproduction signal to increase the C/N ratio. Further, by using thematerial obtained by adding Cu to Zn, it is possible to fully improvethe recording sensitivity of the recording layer. Additionally, sinceboth Zn and Cu are pollution-free materials, it is possible to minimizeimpact on a terrestrial environment, e.g. even if used opticalinformation recording media 1 are buried in the earth for disposal. Inthis case, it is preferred to use a material having Cu added in anamount not less than 1 at % and less than 50 at %, as the material forforming the second sub-recording film 4 b. Further, to improve thesmoothness of the upper surface of the second sub-recording film 4 b, itis preferred to use a material having Cu added in an amount not lessthan 10 at % and less than 50 at %. Moreover, to further improve thesmoothness of the surface of the same, it is preferred to use a materialhaving Cu added in an amount not less than 16.5 at % and not more than36 at %. In the embodiment of the present invention, for example, theatomic percentage of Zn to the whole material for forming the secondsub-recording film 4 b is defined to be 75 at %, and that of Cu, whichis added, to the same is defined to be 25 at %.

As the thickness of the first sub-recording film 4 a and that of thesecond sub-recording film 4 b (the total thickness of the recordinglayer 4) are increased, the smoothness of the upper surface of the firstsub-recording film 4 a located closer to the incidence plane of thelaser beam L is reduced to cause an increase in the noise level of thereproduction signal and degrade the recording sensitivity of therecording layer 4. In this case, when the thickness of the recordinglayer 4 exceeds 50 nm, the recording sensitivity thereof is so reducedthat it can be difficult to use the medium 1 as the optical informationrecording medium. Meanwhile, when the total thickness of the recordinglayer 4 is less than 2 nm, the amount of change in opticalcharacteristics of the recording layer 4 before and after recording ofdata thereon is decreased to decrease the C/N ratio, which makes itdifficult to normally reproduce record data. Therefore, preferably, thetotal thickness of the recording layer 4 is defined to be not less than2 nm and not more than 50 nm, and more preferably, it is defined to benot less than 2 nm and not more than 30 nm. It should be noted that inthe optical information recording medium 1 according to the presentembodiment, to further decrease the noise level of a reproduction signaland obtain the signal as one having a higher C/N ratio, the twosub-recording films 4 a and 4 b are formed such that the total thicknessof the recording layer 4 is not less than 5 nm and not more than 15 nm.

Although the respective thicknesses of the sub-recording films 4 a and 4b are not particularly limited, to fully improve the recordingsensitivity of the recording layer 4, and at the same time sufficientlychange the optical characteristics of the recording layer 4 before andafter recording of data thereon, it is preferable to form thesub-recording films 4 a and 4 b such that each of them has a thicknessnot less than 2 nm and not more than 30 nm. Further, to moresufficiently change the optical characteristics of the recording layer 4before and after recording of data thereon, it is preferable to definethe respective thicknesses of the sub-recording films 4 a and 4 b suchthat the ratio between the thickness of the first sub-recording film 4 aand that of the second sub-recording film 4 b (thickness of the firstsub-recording film 4 a/thickness of the second sub-recording film 4 b)is not less than 0.2 and not more than 5.0. In the embodiment of thepresent invention, the recording layer 4 is formed, for example, suchthat the total thickness thereof becomes equal to 10 nm by defining thethickness of the first sub-recording film 4 a to be 5 nm, and that ofthe second sub-recording film 4 b to be 5 nm.

The light transmitting layer 6 functions as an optical path of the laserbeam when data is recorded or reproduced, and at the same timephysically protects the recording layer 4 and the first dielectric layer5 a. The light transmitting layer 6 is formed of a resin material, suchas a ultraviolet-curing resin or an electron beam-curing resin, suchthat it has a thickness not less than 1 μm and not more than 200 μm(preferably, not less than 50 μm and not more than 150 μm: e.g. 100 μm).In this case, when the light transmitting layer 6 has a thickness ofless than 1 μm, it becomes difficult to protect the recording layer 4and the first dielectric layer 5 a, whereas when the light transmittinglayer 6 has a thickness of more than 200 μm, it becomes difficult toform a light transmitting layer 6 whose parts (particularly, parts inthe radial direction) have a uniform thickness. Further, when the thicklight transmitting layer 6 is formed of a material different from thematerial for forming the substrate 2, warpage of the optical informationrecording medium 1 can be caused by thermal expansion, thermalshrinkage, or the like thereof. It should be noted that the method offorming the light transmitting layer 6 includes a method of applying aresin material (on the first dielectric layer 5 a) by the spin coatingmethod or the like, and then curing the same, a method of affixing asheet material formed of a light-transmitting resin to the firstdielectric layer 5 a by an adhesive or the like. However, to preventattenuation of the laser beam L, it is preferable to employ the spincoating method which does not add the adhesive layer in the medium 1.

When the optical information recording medium 1 is manufactured, first,a stamper for molding a substrate is set in a mold mounted to aninjection molder. Then, the temperature of a polycarbonate resin, andthe temperature of the mold are set to approximately 360 degree Celsiusand approximately 120 degree Celsius, respectively, and at the sametime, other molding conditions, such as a clamping force, a cooling timeperiod, and the like, are set. The substrate 2 is formed by injectionmolding under these conditions. Then, the reflective layer 3 having athickness of approximately 100 nm is formed on the upper surface of thesubstrate 2 by the vapor phase growth method (the vacuum depositionmethod, the sputtering method, or the like: in the present case, e.g.the sputtering method) using chemical species containing Ag as the maincomponent. Next, the second dielectric layer 5 b having a thickness ofapproximately 25 nm is formed by the vapor phase growth method usingchemical species mainly composed of a mixture of ZnS and SiO₂ in amanner covering the reflective layer 3. Then, the second sub-recordingfilm 4 b having a thickness of approximately 5 nm is formed by the vaporphase growth method using the material (chemical species) containing Znas the main component and having Cu added thereto, in a manner coveringthe second dielectric layer 5 b. Since the second sub-recording film 4 bis formed by using the material obtained by adding Cu, the upper surfaceof the second sub-recording film 4 b is made even flatter or smoother(the smoothness of the upper surface of the second sub-recording film 4b is ensured), compared with a case where a material having no Cu addedthereto is used.

Then, the first sub-recording film 4 a having a thickness ofapproximately 5 nm is formed by the vapor phase growth method using thematerial (chemical species) containing Si as the main component in amanner covering the second sub-recording film 4 b. In forming the firstsub-recording film 4 a, since the upper surface of the secondsub-recording film 4 b has been formed flat, the upper surface of thefirst sub-recording film 4 a is also formed flat similarly to the uppersurface of the second sub-recording film 4 b. After that, the firstdielectric layer 5 a having a thickness of approximately 25 nm is formedby the vapor phase growth method using chemical species mainly composedof a mixture of ZnS and SiO₂, in a manner covering the firstsub-recording film 4 a. It should be noted that it is preferable to formthe reflective layer 3, the second dielectric layer 5 b, the secondsub-recording film 4 b, the first sub-recording film 4 a, and the firstdielectric layer 5 a, successively on the substrate 2 by using asputtering machine having a plurality of sputtering chambers, byadjusting layer-forming conditions on a chamber-by-chamber basis asrequired. Subsequently, the light transmitting layer 6 having athickness of approximately 100 μm is formed on the first dielectriclayer 5 a by applying e.g. an acrylic-based ultraviolet-curing resin (oran epoxy-based ultraviolet-curing resin) in a manner covering the firstdielectric layer 5 a by the spin coating method, and then curing thesame. In forming the light transmitting layer 6, conditions for spincoating (rotational speed, rate of change in the speed, a time periodtaken until the rotation is stopped, etc.) are adjusted, as required, soas to form the layer 6 such that it has a uniform thickness(particularly, in the radial direction). Further, to form the lighttransmitting layer 6 such that it has a thickness of approximately 100μm, it is preferable to use a resin material (ultraviolet-curing resin,in the present case) whose viscosity is high to some extent. Thus, theoptical information recording medium 1 is completed.

Next, the principles of recording of record data by the opticalinformation recording medium 1 will be described with reference todrawings.

First, the laser beam L adjusted to a recording power (e.g. ofapproximately 5.0 mW at the upper surface of the light transmittinglayer 6), having a wavelength (λ) of 405 nm, is emitted from anobjective lens having a numerical aperture (NA) of 0.85, and irradiatedonto the optical information recording medium 1. At this time, in anarea of the recording layer 4 to which the laser beam L is irradiated,an element (Si, in the present case), which is the main component of thefirst sub-recording film 4 a, and an element (Zn, in the present case),which is the main component of the second sub-recording film 4 b, aremixed with each other, to form the recorded portions M (one of which isshown in FIG. 1). It should be noted that although FIG. 1 shows a stateof the whole area irradiated with the laser beam L, in which the firstsub-recording film 4 a and the second sub-recording film 4 b are totallymixed in the direction of thickness to form the recorded portions M, itis also possible to form the recorded portions M such that record datacan be normally reproduced therefrom (such that the record data issufficiently readable), even if the first and second sub-recording films4 a and 4 b are partially mixed with each other in the vicinity of theinterface therebetween. In this case, portions remaining in the state ofthe first sub-recording film 4 a and the second sub-recording film 4 bbeing layered (hereinafter also referred to as the “layered portions” or“unrecorded portions”), and the recorded portions M are largelydifferent in optical characteristics. Hence, when the laser beam Ladjusted to a reproduction power is irradiated to the layered portionand the recorded portions M, respective values of reflectance exhibitedby the two kinds of portions are very different from each other.Therefore, by detecting the difference in the reflectance, it isdetermined whether or not each recorded portion M exists, and based onthe determination, the recording/reproducing apparatus reproduces(reads) record data.

In the present optical information recording medium 1, the recordinglayer 4 is sandwiched by the first dielectric layer 5 a and the seconddielectric layer 5 b, and hence even when the first sub-recording film 4a and the second sub-recording film 4 b are heated by the irradiation ofthe laser beam L to the extent that they are mixed with each other, itis possible to prevent thermal deformations of the substrate 2 and thelight transmitting layer 6. This makes it possible to prevent a rise inthe noise level, a decrease in the C/N ratio, and deterioration ofjitter characteristics. Furthermore, since the first sub-recording film4 a is made of the material containing Si as the main component, and thesecond sub-recording film 4 b is made of the material containing Zn asthe main component, it is possible to sufficiently change the opticalcharacteristics of portions of the recording layer 4, which are formedinto the recorded portions M, before and after recording of record datathereon. As a result, it can be positively detected whether or not eachrecorded portion M exists, thereby making it possible to reliablyreproduce record data.

Next, the relationship between the amount of Cu added to the material(material containing Zn as the main component) for forming the secondsub-recording film 4 b, and the noise level will be described withreference to FIGS. 2.

As described hereinbefore, if Cu is added to the material (materialcontaining Zn as the main component) for forming the secondsub-recording film 4 b, it is possible to improve the smoothness of theupper surface of the second sub-recording film 4 b formed by the vaporphase growth method. Therefore, it is possible to decrease the level ofnoise contained in the reproduction signal (noise level of thereproduction signal). In this case, the present inventors have confirmedthat the noise level at least in a frequency band of 4.1 MHz to 16.5 MHzis reduced by addition of Cu. More specifically, as shown in FIG. 2,when Cu is not added to the material for forming the secondsub-recording film 4 b, which contains Zn as the main component (when Cuis added in an amount of 0 at %), the noise level (for example, thenoise level in the vicinity of 4.2 MHz was measured, while rotating theoptical information recording medium 1 at a linear velocity of 5.3 m/s)of the reproduction signal is 51.0 dBm. In contrast, when Cu is added tothe material containing Zn as the main component in an amount of 10 at%, the noise level is lowered to −52.1 dBm. Further, when Cu is added inamounts of 16.5 at %, 27.9 at %, and 35.9 at %, the noise level islowered to −57.7 dBm, −60.8 dBm, and −57.7 dBm, respectively. As to thelowering of the noise level, even when Cu is added up to a little lessthan 50.0 at % (less than 50.0 at %), it is considered that the noiselevel is also lowered to −52 dBm or lower.

Therefore, by adding Cu in an amount not less than 10 at % and less than50 at % to the material containing Zn as the main component, it ispossible to lower the noise level to approximately −52 dBm. Further, byadding Cu in an amount not less than 16.5 at % and not more than 36 at %to the material containing Zn as the main component, it is possible tolower the noise level to approximately −58 dBm. Therefore, by formingthe second sub-recording film 4 b using the material containing Zn asthe main component and having Cu added thereto in an amount not lessthan 10 at % and less than 50 at %, it is possible to manufacture theoptical information recording medium 1 which is capable of normallyreproducing record data (generating the reproduction signal reduced innoise level). Further, by defining the amount of Cu to be added as notless than 16.5 at % and not more than 36 at %, it is possible to furtherlower the noise level.

Next, a description will be given of the relationship between thethickness (layer thickness) of the recording layer 4 and the C/N ratiowith reference to FIG. 3.

As described above, by reducing the layer thickness of the recordinglayer 4 (the sum of the thickness of the first sub-recording film 4 aand the thickness of the second sub-recording film 4 b), it is possibleto enhance the smoothness of the upper surfaces of the first and secondsub-recording films 4 a and 4 b, thereby making it possible to increasethe C/N ratio. However, when the layer thickness of the recording layer4 is too small, the amount of change in optical characteristics of therecording layer 4 before and after recording of data thereon becomes toosmall, causing a reduced C/N ratio. This makes it difficult to normallyreproduce record data. On the other hand, when the layer thickness ofthe recording layer 4 is too large, a crack can be produced in theoptical information recording medium 1 due to internal stresses of thefirst and second sub-recording films 4 a and 4 b, and the smoothness ofthe upper surfaces of the first and second sub-recording films 4 a and 4b is reduced to increase the noise level, which results in the reducedC/N ratio. Further, when the layer thickness of the recording layer 4 istoo large, the recording sensitivity of the recording layer 4 can belowered. Therefore, it is required to define the layer thickness of therecording layer 4 in view of such circumstances.

More specifically, as shown in FIG. 3, when the layer thickness of therecording layer 4 is more than 30 nm, the C/N ratio becomes lower than31 dB to make it difficult to normally reproduce record data, whereaswhen the layer thickness of the recording layer 4 is not more than 30nm, the C/N ratio becomes not less than 31 dB, which makes it possibleto perform reliable reproduction of record data. Further, when the layerthickness of the recording layer 4 is not more than 24 nm, the C/N ratiobecomes not less than 35 dB, and when the same is not more than 12 nm,the C/N ratio becomes not less than 45 dB, which makes it possible toperform more reliable reproduction of record data. In this case,however, when the layer thickness of the recording layer 4 is less than2 nm, the optical characteristics of the recording layer 4 are hardlychanged between before and after recording of the record data whichmakes it difficult to perform normal reproduction of record data.Further, when the layer thickness of the recording layer 4 is more than30 nm, the smoothness of the upper surfaces of the first and secondsub-recording films 4 a and 4 b is reduced, resulting in the increasednoise level. Furthermore, when the layer thickness of the recordinglayer 4 is more than 30 nm, a crack is produced in the recording layer 4due to the internal stresses of the first and second sub-recording films4 a and 4 b, when the optical information recording medium 1 is heatedor cooled, or when a force for bending the medium 1 is applied to themedium 1. Additionally, it has also been confirmed that when the layerthickness of the recording layer 4 is more than 30 nm, the recordingsensitivity of the recording layer 4 is reduced.

Therefore, to reliably reproduce record data while preventing productionof a crack and reduction of the recording sensitivity, it is preferableto form the first sub-recording film 4 a and the second sub-recordingfilm 4 b such that the layer thickness of the recording layer 4 is notless than 2 nm and not more than 30 nm. Further, to more reliablyreproduce record data by improving the C/N ratio, it is preferable toform the first sub-recording film 4 a and the second sub-recording film4 b such that the layer thickness of the recording layer 4 is not lessthan 2 nm and not more than 24 nm (more preferably, not less than 2 nmand not more than 12 nm).

As described hereinabove, according to the optical information recordingmedium 1, the first sub-recording film 4 a is formed by using a materialcontaining Si as the main component, and the second sub-recording film 4b is formed in the vicinity of the first sub-recording film 4 a by usinga material containing Zn as the main component and having Cu addedthereto. The addition of Cu makes it possible to improve the smoothnessof the upper surface of the second sub-recording film 4 b, therebyconsiderably lowering the noise level. Therefore, the C/N ratio can beenhanced, whereby record data can be reliably reproduced.

According to the optical information recording medium 1, the secondsub-recording film 4 b is formed by using the material having Cu addedthereto in an amount not less than 10 at % and less than 50 at %, it ispossible to decrease the noise level to approximately −52 dBm or lower.This makes it possible to provide the optical information recordingmedium 1 capable of reliably reproducing record data.

Furthermore, according to the optical information recording medium 1,the second sub-recording film 4 b is formed by using the material havingCu added thereto in an amount not less than 16.5 at % and not more than36 at %, whereby it is possible to decrease the noise level toapproximately −58 dBm or lower. As a result, it is possible to morereliably reproduce record data.

Further, according to the optical information recording medium 1, therecording layer 4 is constructed by forming the first sub-recording film4 a and the second sub-recording film 4 b such that they are in contactwith each other, whereby when the laser beam L adjusted to the recordingpower is irradiated to the recording layer, the first sub-recording film4 a and the second sub-recording film 4 b can be easily mixed with eachother to thereby form the recorded portions M.

Further, according to the present optical information recording medium1, since the total of the thickness of the first sub-recording film 4 aand that of the second sub-recording film 4 b is defined to be not lessthan 2 nm and not more than 30 nm, it is possible to fully improve theC/N ratio, which enables record data to be more reliably reproduced.

Moreover, according to the present optical information recording medium1, the recording layer 4 is configured to be sandwiched between thefirst dielectric layer 5 a and the second dielectric layer 5 b, it ispossible to prevent thermal deformations of the substrate 2 and thelight transmitting layer 6 during irradiation of the laser beam L(during formation of the recorded portions M). Further, since corrosionof the recording layer 4 can be prevented, it is possible to storerecord data for a long time period in a manner such that the record datacan be normally reproduced.

Furthermore, according to the optical information recording medium 1,the reflective layer 3 is formed between the substrate 2 and the seconddielectric layer 5 b, whereby the effect of multi-beam interference isfurther increased. This makes it possible to further increase thedifference in optical reflectance between the recorded portions M andthe unrecorded portions, whereby it is possible to more reliablyreproduce record data.

It should be noted that the present invention is by no means limited tothe aforementioned embodiment. For example, although in theabove-described embodiment, the description has been given of theexamples in which the first sub-recording film 4 a and the secondsub-recording film 4 b are arranged adjacent to each other in thedirection of thickness of the optical information recording medium 1,this is not limitative, but the construction of the recording layer 4can be changed as required, so long as it is configured to be capable offorming an area for mixing the first sub-recording film 4 a and thesecond sub-recording film 4 b with each other, when the laser beam Ladjusted to the recording power is irradiated to the recording layer 4.More specifically, for example, the recording layer 4 can also be formedby interposing one or more very thin dielectric layers or the likebetween the first sub-recording film 4 a and the second sub-recordingfilm 4 b, or alternatively it can be formed by interposing a layer madeof a mixture of materials for forming the first sub-recording film 4 aand the second sub-recording film 4 b, between the sub-recording films 4a and 4 b. Further, although in the above-described embodiment, thedescription has been given, by way of example, of the opticalinformation recording medium 1 which includes the recording layer 4comprised of two recording films, i.e. the first sub-recording film 4 aand the second sub-recording film 4 b, this is not limitative, but therecording layer according to the present invention can be configured tohave not only the sub-recording films 4 a and 4 b but also one or moresub-recording films which are formed of a material containing an elementselected from the group consisting of Si, Ge, C, Sn, Zn, and Cu, as themain component.

Further, although in the above-described embodiment, the description hasbeen given of the optical information recording medium 1 which has thefirst sub-recording film 4 a corresponding to the first recording filmin the present invention formed toward i.e. closer to the lighttransmitting layer 6, and the second sub-recording film 4 bcorresponding to the second recording film in the present inventionformed toward i.e. closer to the substrate 2, this is not limitative,but it is possible to employ the construction in which the firstsub-recording film 4 a is formed toward i.e. closer to the substrate 2and the second sub-recording film 4 b is formed toward i.e. closer tothe light transmitting layer 6. Further, although in the above-describedembodiment, the description has been given of the opticalinformation-recording medium 1, including the first dielectric layer 5 aand the second dielectric layer 5 b, this is not limitative, but theoptical information recording medium according to the present inventionencompasses optical information recording media which do not have one orany of the first dielectric layer 5 a and the second dielectric layer 5b. Furthermore, although in the above-described embodiment, thedescription has been given of the optical information recording medium1, including the reflective layer 3, this is not limitative, but theoptical information recording medium according to the present inventionencompasses optical information recording media which do not have thereflective layer 3. Furthermore, although in the above-describedembodiment, the thicknesses of the respective layers are described onlyby way of examples, and this is not limitative, but of course they canbe changed as required.

1. An optical information recording medium for recording and reproducing record data, comprising: a substrate; a recording layer formed on the substrate, for having a laser beam irradiated thereto for recording and reproduction of the record data; and a light transmitting layer formed in a manner covering the recording layer, wherein the recording layer includes a first recording film formed of a first material containing Si as a main component, and a second recording film formed of a second material containing Zn as a main component and having Cu added thereto, the second recording film being formed in the vicinity of the first recording film, and wherein the laser beam is irradiated to the recording layer from a light transmitting layer side.
 2. An optical information recording medium as claimed in claim 1, wherein the second material has Cu added thereto in an amount not less than 10 at % and less than 50 at %.
 3. An optical information recording medium as claimed in claim 2, wherein the second material has Cu added thereto in an amount not less than 16.5 at % and not more than 36 at %.
 4. An optical information recording medium as claimed in claim 1, wherein the recording layer is configured such that the first and second recording films are in contact with each other.
 5. An optical information recording medium as claimed in claim 1, wherein the recording layer is formed such that a total of a thickness of the first recording film and a thickness of the second recording film is not less than 2 nm and not more than 30 nm.
 6. An optical information recording medium as claimed in claim 1, including a first dielectric layer formed between the light transmitting layer and the recording layer, and a second dielectric layer formed between the substrate and the recording layer.
 7. An optical information recording medium as claimed in claim 6, including a reflective layer formed between the substrate and the second dielectric layer. 